According to the Officer Down Memorial Page, 71 officers in 10 local, state and national agencies, were killed when the Twin Towers collapsed. Others have died after 2001 due to illnesses they developed after working in the dust and debris left behind in the wreckage in New York.

New York Mayor Michael Bloomberg (C) attends the funeral mass for police officer Moira Smith February 14, 2002 at St. Patricks Cathedral in New York City. Smith was one of 23 NYPD officers killed during the September 11, 2001 attacks on the World Trade Center.
(Spencer Platt/Getty Images)